A medical ventilator is an apparatus to give artificial respiration. A volume ventilator delivers a given volume regardless of the pressure in the breathing system in which a patient is connected.
Present day mechanical ventilators are typically used in one of five manually selected modes. A first is a control mode in which the patient is totally inactive and patient inhalation is controlled solely by the ventilator. Under such circumstances, exhalation is always passive with no participation by the ventilator except to open a conventional expiratory valve. A second mode is an assist-control technique in which an inspiratory cycle is triggered either by the patient's inspiratory effort, or by the machine, whichever occurs first. Once triggered, the ventilator delivers a selected tidal volume. In a purely assist mode, the inspiratory cycle is triggered only by the patient's inspiratory effort. The machine does not initiate the inspiration automatically. Once the inspiratory cycle is started, the ventilator delivers the full tidal volume. A fourth mode is known as spontaneous in which the ventilator supplies only the breathing gas, and the patient breathes on his or her own without any assistance. A fifth mode is referred to as IMV (Intermittent Mandatory Ventilation). For this mode, the ventilator is set to a very low rate, such as one or two breaths per minute. The ventilator usually delivers as in the assist mode. The reason for the IMV is to assure at least the set amount of ventilation if the patient should decrease breathing spontaneously or should stop breathing altogether.
In present day clinical practice and physiology, ventilation is thought of in terms of minute volume ventilation as a basic parameter. A derivation of minute volume requires a calculation of, for example, tidal volume and respiratory rate.
The presently available and commercially used ventilators are all mechanical devices which are incapable of performing such calculations and are not equipped for the adjustment of the mechanical functions according to the calculated results. As a consequence, prior art ventilators require manual settings of the mechanical functions such as flow, expiration time, inspiration time and the like, rather than the familiar physiological parameters, such as minute volume ventilation. Resultingly, the prior art ventilators have proven very confusing and cumbersome for the average nurse and physician.